1. Field of the Invention
The present invention relates to novel thermoplastic powder compositions based on polyamides and/or polyetheresteramides which are especially adapted for coating metal substrates without the requirement for any adhesion primer.
2. Description of the Prior Art
The polyamides are commonly employed for coating metal substrates, especially because of their good mechanical properties such as abrasion resistance, impact strength, and the like, and chemical inertness towards many materials such as hydrocarbons, bases, inorganic acids, etc.
It is known to this art, however, that the adhesiveness of the polyamides to metals is insufficient because of the poor wetting ability of the polyamides in the molten state, which does not permit them to penetrate well into the roughness and interstices of the metal in order to provide an adhesiveness of mechanical order.
Furthermore, when a uniform deposit of polyamide powder is provided over an entire metal surface and when this deposit is heated until it melts at the appropriate temperature, the molten polyamide film shrinks and coheres together in droplets which can fall or drop off of the metal substrate.
To overcome this drawback the metal substrate is coated with an undercoat, designated an adhesion primer, intended to ensure the bonding and the mechanical anchoring of the polyamide powder. The adhesion primer employed is generally based on thermosetting resins and is applied in the form of a powder, or in solution or suspension in organic solvents.
Additional equipment must therefore be provided for possible removal of the solvents and for curing the primer before the substrate thus coated is clad with the polyamide powder. Moreover, the curing and/or the drying of the primer results in an appreciable increase in the duration of the coating operations and, hence, in their cost.
French Patent No. 72/41,484 describes powder compositions based on polyamide containing blocked phenols that do not require the use of an adhesion primer. However, the application conditions are intricate and it is impossible to obtain high-performance adhesiveness results, although they are indeed improved.